The term “DC converter” or simply “converter” is used herein to include all forms of DC converters, also referred to as DC-to-DC converters, DC or DC-to-DC regulators, switch mode regulators, converters, or power supplies, etc., whether or not they include an isolating transformer. The term “regulator” is used herein to refer to a converter without an isolating transformer, and a converter with an isolating transformer is referred to herein as an “isolated converter”.
Controllers in accordance with this invention are intended to be particularly suitable for a forward or buck converter, and the invention is accordingly described in this context. However, the invention is not limited in this respect and the controller may also be used for other converter topologies and/or in other applications. A buck regulator is a step-down converter which converts an input voltage to a lower magnitude output voltage; in an isolated buck converter the relationship between the input and output voltages is modified by the transformation ratio of the isolating transformer.
A DC converter can be controlled using open loop and/or closed loop control which may involve analog and/or digital techniques. For example, PWM (pulse width modulation) can be used in a closed loop control arrangement in which the converter output voltage is compared with a reference voltage to produce an error signal, and the error signal is used to control the PWM.
A digital PWM control arrangement, in which there is at least one digital signal in dependence upon which the PWM is controlled, may be desirable. However, digital PWM involves a disadvantage in that the resolution or step size for regulating the converter output voltage is dependent upon the resolution with which the PWM pulse width can be adjusted, so that very high resolution of the PWM pulse width may be required to provide a desired regulation of the converter output voltage. This, in turn, involves the disadvantage of a very high clock frequency for the PWM control arrangement.
These difficulties are exacerbated when the buck converter operates with a small duty cycle (i.e. when the PWM pulse width is small compared with the PWM period) to provide a large step-down from an input voltage which is much larger than the output voltage. Although the need for operation with a small duty cycle can be reduced in some cases by taking advantage of the transformation ratio of the transformer of an isolated converter, this is not the case where the converter is required to operate from a wide range of possible input voltages.
Although it would be possible to reduce the switching frequency (i.e. increase the PWM period) of the converter to reduce these difficulties, this is undesirable because it would also significantly increase the size of filter components required.
Accordingly, there is a need for an improved digital controller which is suitable for controlling DC converters.